1. Field of the Invention
The present invention generally relates to the fabrication of devices and photomasks with optical, e-beam and x-ray exposures. In particular, the present invention relates to forming resist images (e.g., high resolution resist images) without profile distortion near the interface between resist and underlying structure beneath the resist.
2. Description of the Related Art
In the microelectronics industry, as well as other industries involving, for example, the making of micromachines, magnetoresistive heads, etc., there is a continued desire to reduce the size of microstructural devices. In particular, the microelectronics industry wishes to provide a greater amount of microelectronic circuitry within smaller electronic chips.
Reducing the size of microelectronic devices requires improved photolithographic techniques. Photolithographic techniques involve not only the formation of photo-imaged patterns on a substrate, such as a silicon wafer, but also the making of photomasks, usually called masks, that provide the patterns for the photo-imaging process.
New photolithographic techniques involve a movement in the industry to shorten the wavelength of exposure sources and use deep ultraviolet (UV) radiation and chemically amplified resists. Such photoresists offer the potential of forming images of smaller features than may be possible at longer wavelength exposure. As is recognized by those in the art, “deep UV radiation” refers to exposure radiation having a wavelength in the range of 350 nm or less, more typically in the range of 300 nm or less such as radiation provided by a KrF excimer laser light (248 nm) or an ArF excimer laser light (193 nm).
However, a significant disadvantage with chemically-amplified resists is their frequent sensitivity to the environment as well as the underlying substrate, which can result in reduced resolution of the resist relief image. In particular, resolution problems often occur upon coating onto substrates such as TiN, SiN, and SiON.
Deposition and processing of resists on nitride substrates is required for many microelectronic device fabrications. However, developed images of many current resists applied on TiN or other nitride substrates often will exhibit “footing” for positive tone resist (or “undercut” for the negative tone resist), where the resist fails to clear during development resulting in an upwardly tapering relief image sidewall.
The making of masks, usually a chrome on glass or quartz process, is similar to that of the conventional making of patterned silicon wafers for microelectronic circuits. A mask material (e.g., a chrome layer), is covered with a radiation-sensitive and chemically-amplified resist layer, which is then exposed to a pattern of imaging radiation. The resist layer is subsequently developed by contact with a developer (e.g., an aqueous alkaline developer), to selectively remove portions of the resist layer according to the pattern of imaging radiation. The pattern is subsequently transferred to an underlying substrate by etching those areas of the substrate, which have been exposed by the selective removal of portions of the resist layer. After transferring the pattern to the substrate, the remaining portions of the overlying resist layer are removed.
In the case where the underlying substrate is highly-reflective of the imaging radiation, reflection of the patterned imaging radiation back from the surface of the highly-reflective substrate into the overlying resist layer can cause a loss of fidelity in the transfer of the desired pattern to the underlying substrate. To prevent this loss of fidelity, a thin anti-reflective coating, such as a thin oxide layer, is formed between the resist layer and the underlying substrate.
In some masking processes, however, the inventors of the present invention have observed that such a thin oxide layer can also cause a loss of fidelity in the overlying resist layer by “poisoning” the resist layer, which causes footing. Footing is a condition in which the sidewalls of patterned resist layer do not meet the underlying substrate at a sharp well-defined angle. Instead, the base of the sidewall protrudes over the underlying substrate, in a manner similar to a foot protruding from the bottom of a leg.
For example, FIG. 5 illustrates a lithographic structure 50 in which a resist layer 56 is formed on a mask layer 52b (e.g., chrome plate), which was formed on a substrate 52a As illustrated in FIG. 5, during patterning, footing has occurred in portions 56′ of the resist layer 56.
This footing, of course, degrades the transfer of the desired pattern from the overlying resist layer to the underlying substrate. This footing problem is similar to the nitride poisoning of the resist in device fabrication of Si wafers.
One conventional masking system attempts to resolve the footing problem on a titanium nitride substrate by using a thick layer (i.e., from 450 to 1200 Å) of an organic antireflective coating. With other substrate materials, however, such a thick organic antireflective coating can produce significant footing. It is thought that such footing occurs when the overlying thick resist layer is significantly eroded by subsequent etching processes. It has also been observed that a thinner antireflective coating does not cure the footing problem in these other masking systems.
Thus, there remains a need in mask fabrication to overcome the footing problem of an overlying resist layer formed on an antireflective coating of an inorganic oxide with an underlying metal substrate.